Resistance variable memory is a RAM that has electrical resistance characteristics that can be changed by external influences. The basic component of a resistance variable memory cell is a variable resistor. The variable resistor can be programmed to have high resistance or low resistance (in two-state memory circuits), or any intermediate resistance value (in multi-state memory circuits). The different resistance values of the resistance variable memory cell represent the information stored in the resistance variable memory circuit. The advantages of resistance variable memory are the simplicity of the circuit, leading to smaller devices, the non-volatile characteristic of the memory cell, and the stability of the memory states.
FIG. 1 shows a cross-section of a conventional resistance variable memory device. This resistance variable memory device is a Type GRAD (one resistor, one diode) memory device. It includes a word line (N type region) 102 in substrate 100, a plurality of P+ regions 104 and N+ regions 106, wherein word line 102 and P+ region 104 constitute a diode. A dielectric layer 114 is formed over substrate 100. A plurality of memory units 107 are set in dielectric layer 114, wherein each memory unit 107 includes a flat plate bottom electrode 108, a flat plate top electrode 110, and a resistive film 112, which may be formed of one or more layers, between the flat plate bottom electrode 108 and the flat plate top electrode 110. Word line contact via 116 is formed in dielectric layer 114. One end of word line contact via 116 is electrically connected to N+ region 106; the other end is electrically connected to a conducting line 120 on the surface of dielectric layer 114 so that the word line 102 can electrically connect with external circuits. Furthermore, there is a bit line 118 formed on dielectric layer 114 for electrically connecting with top electrode 110 of the memory unit 107.
A second example of a conventional resistance variable memory device is a Type 1R1T (one resistor one transistor) memory device illustrated in FIG. 2. This device includes a plurality of N+ regions 202 and 204 in substrate 200. A dielectric layer 220 is formed over substrate 200. Dielectric layer 220 includes a plurality of memory units 207, a plurality of gate structures (word lines) 212 and a plurality of contact vias 214 and 216. Each memory unit includes a flat plate bottom electrode 206, a flat plate top electrode 208 and a resistive film 210; which may be formed of one or more material layers, each memory unit is set on the surface of a respective N+ region. Gate structure 212 and N+ regions 202 and 204 constitute a transistor. Contact vias 214 and 216 are electrically connected to the gate structure 212 and the common line 204, respectively, so that the gate structure 212 and the common line 204 can connect with the external circuits. Furthermore, there is a bit line 218 formed on dielectric layer 220 for electrically connecting with the flat plate top electrode 208 of the memory unit 207.
Unfortunately, the metal-insulator-metal (MIM) structure with a resistive film or insulting oxide sandwiched between two flat metallic electrode plates as disclosed in FIGS. 1 and 2 does not provide stable and reproducible switching and does not provide memory properties in a controlled manner, as the conduction path between the elements can occur anywhere in the resistive film or insulating oxide between the top and bottom electrodes. The random and unpredictable conduction path between the elements is believed to be created by random and unpredictable defect sites in the deposited film.
There is needed, therefore, an alternative apparatus for improving and controlling the conduction path between the electrodes in a resistance variable memory device to form large arrays of memory devices based on the resistance switching phenomenon.